The study of 84 cereals by the Environmental Working Group said Kellogg's Honey Smacks, Post Golden Crisp and General Mills Wheaties Fuel rank as the worst offenders based on the Twinkie standard.
Forty-four other cereals have more sugar in a cup than three Chips Ahoy cookies, the study found.
"Most parents would never serve dessert for breakfast, but many children's cereals have just as much sugar or more," Jane Houlihan, senior vice president of research for the EWG, said in a release from the group Wednesday. "As a mom of two, I was stunned to discover just how much sugar comes in a box of children's cereal."
Sugar makes up more than a third of the cereal by weight in almost three dozen of the brands studies, the group said.
For the cereal companies, it's about selling products, Marion Nestle, a nutrition and food studies professor at New York University, said.
"The cereals on the EWG highest-sugar list are among the most profitable for their makers, who back up their investment with advertising budgets of $20 million a year or more," she said.
"Kids should not be eating sugar for breakfast. They should be eating real food."
About one in five American children is obese, according to the federal Centers for Disease Control and Prevention, which has reported childhood obesity has tripled over the past 30 years.